Apparatus for electro-hydraulic crushing



Nov. 14, 1967 N. G. MAROUDAS APPARATUS FOR. ELECTRO-HYDRAULIC CRUSHING Filed Feb. 1, 1965 United States Patent 3,352,503 APPARATUS FOR ELECTRO-HYDRAULIC CRUSHING Nicholas George Maroudas, London, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Feb. 1, 1965, Ser. No. 429,464 Claims priority, application Great Britain, Feb. 6, 1964, 5,070/ 64 4 Claims. (Cl. 241-301) ABSTRACT OF THE DISCLOSURE A technique and apparatus for crushing materials by electro-hydraulic techniques is characterized by arranging for a spark to be discharged through a liquid containing the material to be crushed, wherein the total time of discharge is not more than about 1 micro-second. This is achieved by designing the apparatus to have an extremely low inductance.

The present invention relates to improvements in electro-hydraulic techniques.

It has long been known that a spark discharge taking place in a liquid causes intense shock Waves to be set up in the liquid and these shock waves may be used for crushing material in the liquid or for forming material in contact with the liquid. Thus, for example electro-hydraulic techniques may be used for crushing materials such as coal, gravel, chalk or glasses and refractory materials such as carbides or oxides, which, for one reason or another, are difiicult to crush by conventional techniques. The technique may also be used for forming ductile materials, such as heet steel and the like, by placing a blank in contact with an evacuated die and subjecting the side of the blank remote from the die to electro-hydraulic impulses from spark discharges in a liquid in contact with the surface of the blank. Under normal circumstances the liquid will usually be water. A well known class of electro-hydraulic machines comprises a capacitance which is charged to a high voltage from some suitable source and is arranged to discharge via an air gap or other gas gap in series with a liquid gap, with the gas gap functioning as a shaping device for the discharge through the liquid gap which creates the working discharge.

The efliciency of the technique in terms of crushing depends upon two main factors, namely the electro-acoustic efi'icency and the comminution efllciency. In order to increase the overall efficiency of the technique, which is at present less eflicient than conventional methods, it is necessary to raise either the electro-acoustic efiiciency or the comminution efiiciency.

It is the object of the present invention to provide means for increasing, primarily the electro-acoustic efiiciency, in electro-hydraulic crushing techniques.

According to the present invention a method of applying electro-hydraulic techniques to crushing comprises discharging essentially all the energy stored in a capacitor through a spark discharge gap in a liquid medium in a time of not more than about one microsecond.

Preferably at least 85% of the energy stored in the capacitor is discharged in a time of 0.5 microsecond.

For a conventional circuit the optimum parameters are related by V L 6) (1) where R is the mean resistance, in ohms, of the liquid gap during discharge L is inductance in microhenries. C is capacitance in microfarads.

3,352,503 Patented Nov. 14, 1967 The peak power output of the spark is given by The duration of the pulse is approximately where T is total duration of the pulse in microseconds.

We have found that electro-acoustic efiiciency is considerably improved when essentially all the energy stored in the capacitor is discharged across the spark gap in a time which is of the order of one microsecond and that preferably at least of the energy in the capacitor should be discharged in 0.5 microsecond. These conditions are amply fulfilled if T is of the order of 0.5 microsecond. The time for peak power output, is related to T and would in such a case be of the order of 0.15 microsecond.

Consideration of the above equations will show that to get a very short discharge time it is necessary to use low inductances and capaictances but that, on the other hand, to obtain a high power it is necessary to use relatively large capacitances and voltages. Thus, the time of discharge depends upon the product of inductance and capacitance Whilst for a given voltage the power depends upon the square root of the ratio of capacitance to inductance. It consequently follows that if the electro-acoustic eificiency is to be improved it is necessary to use a very low inductance circuit.

According to a further aspect of the present invention there is provided electro-acoustic spark discharge crushing apparatus comprising a spark discharge gap in a containing vessel, such gap being defined by a pair of electrodes adapted to be immersed in a liquid medium contained in the said containing vessel, such electrodes being connected via a gas spark gap to opposite plates of at least one capacitor which is adapted to be connected to a source of high potential, the circuit comprising the two spark gaps and the capacitor having an inductance of not more than 0.1 microhenry and the capacitance is such that the energy thereof is discharged in a time of not more than one microsecond.

It follows that with such a low value of inductance a practical value of capacitance will be of the order of 0.1 microfarad, which will give a value for T of 0.5 microsecond. This gives a value for R of 1 ohm and a value as great as this may be obtained in practice.

In practice we have found that there is a lower limit to the capacitance which may be employed in an endeavour to reduce the spark discharge time as, in the first place, a very low capacitance will not be able to store sufiicient energy to disrupt the liquid medium and, secondly, the optimum gap resistance will be further increased.

Thus, the apparatus of the present invention does not differ, in its essential parts, from those conventional in the art but it does dififer fundamentally in the electrical values of the circuit formed by these parts and it is in the arrangement of the conventional parts to achieve the low values of inductance with the consequent low value of spark time that the present invention resides in one of its aspects. Moreover, the detailed form in which the invention is embodied diifers from the forms previously proposed.

In order that the present invention may more readily be understood, one embodiment of the same will now be described with reference to the accompanying drawing which is a cross-sectional diagrammatic view of an electro-hydraulic machine adapted for crushing.

Referring'now to the drawing, the treatment cell comprises a brass tube 1 which is provided with an internal lining 2 of suitable insulating material, for example polythene. Below the lining 2, the tube 1 is provided with a grid 3 and an upfiow of water is arranged to pass through the tube 1 in the direction of the arrow A. It will be appreciated that the efiect of this upflow of water is to sweep out of the portion of the tube, represented in the drawings, any of the crushed material which has been reduced below a specific size which will depend upon the specific gravity of the material and the velocity of water supplied.

A first electrode 4 is provided in the tube 1 and consists of a screw-threaded member engaged with a screwthreaded bore in the tube 1 in such a manner that the electrode passes radially into the tube through the insulating lining 2. The outer end of the electrode 4 is provided with a knurled knob 5 to enable the radial position of the electrode to be adjusted.

The tube 1 is provided with a radial tubular extension 6 diametrically opposite to the electrode 4 and at its outer end this extension 6 carries a flat brass plate 7.

The brass plate 7 forms one plate of the capacitance and the integers 1, 6 and 7 are electrically bonded together and to earth so that the electrode 4 is the earthed electrode. It is important to observe that this arrangement provides for minimum inductance in the circuit and that if, for example, the electrode 4 were connected to the plate 7 by means of a loop or strap of a conducting material such as copper, the increase in inductance would be such that the apparatus would not be within the scope of the present invention.

The extension 6 is also lined with insulating material 8 and as can be seen a Web 9 of insulating material closes the end of the extension 6 adjacent to the tube 1, this web 9 being adapted to support a second electrode 10 which passes therethrough into the tube 1 and which has a hemispherical head 11 located Within the extension 6.

A flat circular plate 12 of insulating material is disposed adjacent to the face of the plate 7 remote from the tube 1 and a further circular plate of brass 13 is applied to the free face of the plate 12 so as to form the other plate of the capacitance, the plate 13 being adapted for connection to a high voltage source. The plate '13 is provided with a centrally located threaded bore in which is housed a threaded tube 14 which passes axially into the extension 6 and has a hemispherical head 15. Gas may be passed into the extension 6 by means of the bore 16 of the tube 14 and, as can be seen, the tube 14 with its head are electrically connected to the plate 13 of the capacitance so that the heads 11 and 15 .form a gas gap for the generation of the shaping discharge. The gas pressure at the gas gap may be adjusted by means of the bore 16.

The operation of the apparatus is extremely simple, and it will be understood that, the flow of water being turned on and a material to be comminuted being present between the electrodes 4 and 10, the plate 13 is charged to a high voltage by means of a generator connected between it and the earthed plate 7. The electrode 4 is connected to the earthed plate 7 and the head 15 is connected to the high voltage plate 13. In consequence, when the potential difierence between the plates 7 and 13 reaches a sufiiciently high value, spark discharges will take place simultaneously between the heads 11 and 15 and between the electrodes 4 and 10. The discharge tak- 4 material therein, which will then be swept away by the stream of water.

If it is desired to crush material to give a relatively fine product from a relatively coarse starting material it may be desirable to provide several stages of crushing. This may readily be achieved by connecting the cells in series so that material crushed in the cell illustrated in the drawing passes upwardly, due to the flow of water, to reach a second cell situated above the first cell Where the particle size of the material is reduced still further.

For efiiciency reasons it may also be desirable to provide the crushing cell in the form of an annulus in which case it will be convenient to provide a plurality of interconnected earthed electrodes extending radially from the inner wall of the annulus with a plurality of separately energized electrodes extending inwardly from the outer wall of the annulus. This arrangement may also be used with the series arrangement above described in order to obtain a helical configuration.

With voltages of between 7 and kilovolts satisfactory crushing is obtained, the gap length being 1-2 ems. with a resistance of the order indicated by Equation 1.

In an experiment using the apparatus shown in the drawings but modified to place 5 capacitors each of .005 t. capacity between the plates 7 and '13 and wherein the internal diameter of the insulation 2 was 2% inches (5.6 cms.), a water spark gap of 12 mms. was used. The capacitors were charged to 38 kv. to give a sparking rate of 30 per minute. The crushing chamber was loaded with 250 gm. of inch gravel and the water flow was 8.5 litres/minute so that gravel crushed to smaller than about 300 microns was washed out of the crushing chamber. The mean of several runs gave gms. of crushed material for 750 sparks (25 minutes) which corresponds to an energy consumption (based on the capacitors) of 39.5 kilowatt hours per tonne for crushing inch gravel to small than 300 microns.

I claim:

1. Electro-acoustic discharge crushing apparatus comprising a containing vessel, a first pair of electrodes defining a spark discharge gap in said containing vessel, such electrodes being adapted to be immersed in a liquid medium contained in the said vessel, a capacitor having first and second plates, a second pair of electrodes defining a gas spark gap, a connection from one of said first pair of electrodes to one of said second pair, connections from the other two electrodes to said capacitor plates, the circuit comprising the two spark gaps and the capacitor having an inductance of not more than 0.1 microhenry and the capacitance being such that the energy thereof is discharged in a time of not more than about one microsecond.

2. The apparatus of claim 1 wherein one of'the first pair of electrodes is electrically insulated from the containing vessel, the other is directly connected to the containing vessel, and the containing vessel is connected directly to one of the plates of the capacitor.

3. The apparatus of claim 2 wherein the containing vessel is formed integrally with one plate of the capacitor.

4. The apparatus of claim 2, wherein said direct connection is in the form of a metallic tube surrounding, but insulated from, said second pair of electrodes.

References Cited UNITED STATES PATENTS 2,163,649 6/1939 Weaver 241-1 X 2,661,784 12/1953 McMillian 24l-1 X 3,181,799 5/ 1963 Wurzburg et al 241-1 3,207,447 9/1965 Whitman 241284 X ANDREW R. JUHASZ, Primary Examiner. 

1. ELECTRO-ACOUSTIC DISCHARGE CRUSHING APPARATUS COMPRISING A CONTAINING VESSEL, A FIRST PAIR OF ELECTRODES DEFINING A SPARK DISCHARGE GAP IN SAID CONTAINING VESSEL, SUCH ELECTRODES BEING ADAPTED TO BE IMMERSED IN A LIQUID MEDIUM CONTAINED IN THE SAID VESSEL, A CAPACITOR HAVING FIRST AND SECOND PLATES, A SECOND PAIR OF ELECTRODES DEFINING A GAS SPARK GAP, A CONNECTION FROM ONE OF SAID FIRST PAIR OF ELECTRODES TO ONE OF SAID SECOND PAIR, CONNECTIONS FORM THE OTHER TWO ELECTRODES TO SAID CAPACITOR PLATES, THE CIRCUIT COMPRISING THE TWO SPARK GAPS AND THE CAPACITOR HAVING AN INDUCTANCE OF NOT MORE THAN 0.1 MICROHENRY AND THE CAPACITANCE BEING SUCH THAT THE ENERGY THEREOF IS DISCHARGED IN A TIME OF NOT MORE THAN ABOUT ONE MICROSECOND. 